Hours Spent Watching TV May Shorten Life Expectancy

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The risk of mortality was significantly higher for participants -- university graduates with a mean age of 37 years -- reporting ≥3 hours per day of television viewing than for those reporting <1 hour per day, researchers found.

However, computer use and time spent driving, both sedentary behaviors, were not significantly associated with higher mortality.

Watching television for many hours a day was associated with an increased risk for premature death in a study of healthy young adults, while computer use and time spent driving showed no significant association with higher mortality.

Study participants who reported watching 3 or more hours of television daily had a twofold increased death risk over 8 years of follow-up, compared with participants who said their viewing time did not exceed 1 hour a day (incidence rate ratio [IRR] 2.04, 95% CI 1.16-3.57), researcher Miguel Angel Martinez-Gonzales, MD, PhD, of the University of Navarra in Pamplona, Spain, and colleagues wrote in the Journal of the American Heart Association.

Sedentary behavior and increased sitting time have been shown in numerous studies to be associated with higher mortality. But this study is among the first to examine the possible impact of different types of sitting behaviors on death risk in healthy adults, Martinez-Gonzales told MedPage Today.

"Our findings suggested that 1 or even 2 hours of television viewing is OK, but spending more than 3 hours watching television is probably not a good idea," he said, adding that as the population ages, watching television and other sedentary behaviors are likely to become more prevalent.

More than Half of Waking Hours Spent Sitting

Most adults in the U.S. are sedentary during more than half of their waking hours, according to a 2008 study. Earlier research has also suggested a link between increased television viewing time and higher mortality, diabetes and cardiovascular disease risk, Martinez-Gonzales said.

Because few studies have examined television screen time and mortality risk independent of other behaviors that involve sitting for long periods, the researchers studied different sedentary behaviors and all-cause mortality in around 13,000 healthy Spanish university graduates (mean age 37) followed for a median of 8.2 years.

All participants completed a baseline questionnaire designed to assess total daily television viewing, computer use and driving time. Each behavior was quantified in 12 categories (ranging from never to more than 9 hours a day) and the exposures were measured separately for weekdays and weekends.

Time spent during weekdays was multiplied by 5 and the time spent during weekends by 2, and results were summed and divided by 7 to calculate the participants' total time per day. The validation study of the questionnaire found that the Spearman correlation coefficient between the energy expenditure estimated through the ratio sedentary lifestyle: physical activity in the questionnaire and that obtained by an objective method (triaxial accelerometer) was -0.578 (95% CI -0.754-minus 0.325).

Questionnaires also assessed medical history, lifestyle, sociodemographic factors and anthropometric measurements. Total physical activity and dietary habits were also assessed through questionnaires completed at study entry. Respondents who reported having cancer, diabetes or cardiovascular disease (n=1206) were excluded from the original study cohort.

No Association Seen for Computer Use or Drive Time

During the follow-up, 97 deaths were registered among the cohort, which was lower than the expected number of deaths (n=128) in a sample of the general Spanish population with the same size, sex, and age distribution, the researchers wrote.

Poisson regression models were used to examine the association between each sedentary behavior and total mortality. All-cause mortality IRRs per 2 hours per day were 1.40 (95% CI 1.06-01.84) for television viewing, 0.96 (95% CI 0.79-1.18) for computer use, and 1.14 (95% CI 0.90-1.44) for driving, after adjustment for age, sex, smoking status, total energy intake, Mediterranean diet adherence, body mass index, and physical activity.

When total sedentary behavior was analyzed as a continuous variable, the IRR per each additional 2 hours per day of exposure was 1.17 (95% CI 1.03-1.33).

The majority of the deaths during follow-up were due to cancer (n=46), followed by noncancer/noncardiovascular causes (n=32) and cardiovascular disease (n=19).

The IRRs for each additional 2 hours each day of television viewing were 1.44 (95% CI 0.87-2.41) for cardiovascular mortality, 1.21 (95% CI 0.73-2.00) for cancer mortality and 1.55 (95% CI 0.96-2.53) for noncardiovascular/noncancer mortality, after adjusting for age, sex, smoking, total energy intake, Mediterranean diet adherence, BMI, computer use, and time spent driving.

The researchers repeated the analyses after excluding all deaths occurring in the first 3 years of follow up (n=35) to check for reverse causality. When this was done (using <1 hour a day as the reference category), the multivariable-adjusted IRRs for all-cause mortality across categories of television viewing were:

1.16 (95% CI 0.56-2.39) for 1 to 2 hours a day

1.75 (95% CI 0.91-3.35) for 2 to 3 hours a day

2.38 (95% CI 1.19-4.74) for >3 hours a day

The P value for linear trend was 0.006.

Excess TV Time Linked to Social Isolation

"Because television viewing is likely to be associated with snacking and consumption of sugar-sweetened beverages, a possible explanation for the association that we found could be a difference in energy intake during television viewing," the researchers wrote.

In an effort to control for this, the researchers repeated their sensitivity analyses to further adjust for snacking and consumption of sugar-sweetened beverages.

Following this adjustment -- again with <1 hour per day as the reference category -- the all-cause mortality IRRs for television viewing were:

1.09 (95% CI 0.61-1.95) for 1 to 2 hours a day

1.45 (95% CI 0.84-2.50) for 2 to 3 hours a day

2.19 (95% CI 1.23-3.91) for >3 hours a day

The P value for linear trend was again 0.006.

More likely explanations for the twofold increase in death risk among participants who watched more than 3 hours of television a day are the fact that television watching tends to be an extremely passive activity, and that people who spend many hours in front of a television each day are often socially isolated.

"Lack of social support is a known risk factor for higher mortality," Martinez-Gonzales said."And when people spend so many hours watching television they are not doing other things that may benefit their health."

He noted that the failure to show an association between total computer or driving time and mortality may be due to differential effects of these activities on cardiometabolic risk factors.

Several other studies have suggested that television viewing time is more directly associated with poor health outcomes, including obesity and type 2 diabetes, than other activities that involve prolonged sitting.

The findings of Martinez-Gonzales and colleagues are similar to those reported in a recent meta-analysis of three prospective studies, which showed a pooled relative risk for all-cause mortality of 1.13 (95% CI 1.07-1.18) for every 2 additional hours a day of television viewing.

Another meta-analysis, which included the studies from the three-study analysis, observed a pooled hazard ratio for all-cause mortality of 1.49 (95% CI 1.14-2.03) for the highest versus lowest television exposure.

Potential study limitations included the fact that the information in the questionnaires was self-reported, and the small number of deaths among the study cohort.

The researchers added that more research is needed to better understand the impact of computer use and driving on mortality and to determine the biological mechanisms behind the observed association between increased TV time and death.